It is common for an assembly such as aircraft landing gear or car suspension system to include a piston and cylinder type hydraulic device such as a shock absorber or actuator.
A piston and cylinder hydraulic device can be provided with an internal seal arranged to confine hydraulic fluid to the device. However, hydraulic fluid can leak beyond the seal if the seal is not capable of withstanding the pressure of the hydraulic fluid; for example, if the seal is damaged during assembly of the hydraulic device.
Such leakage can remain undetected until after the shock absorber has been fitted to the assembly and in some cases until the assembly has been fitted to a larger assembly such as an aircraft or other vehicle. This can result in significant cost and delay, particularly in the case of an aircraft landing gear assembly.